The structural lability in SDS gels of the largest virion polypeptide. (VP1) of type 1 poliovaccine has been successfully exploited as a marker to distinguish neurovirulent from attenuated strains of poliovirus. Similar lesions were not identified in the polypeptides of types 2 and 3 poliovirus vaccine strains. The antigenic characteristics of isolated polypeptides of these viruses are currently under investigation by the technique of radioimmunoprecipitation. Proteins purified by SDS gel electrophoresis have proved to be poor immunogens, thus hindering our attempts to compare these polypeptides directly.